Repairing Heindenhain VRZ 753B + LS803 scales - Page 4
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  1. #61
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    Dear OPW,

    I am sure your unit can be fixed without much work, the fact that it is displaying a single zero on both axes is a good sign.

    I'll have to open up one of my own displays, to see if it has the same part in the same place. However based on the appearance I am guessing that this is a Metal Oxide Varistor (read about it on the internet). This is a sacrificial part, designed to short out and destroy itself to save a circuit from overvoltage. It is easy to get and costs a couple of bucks.

    You wrote, "I have replaced the 12v regulator as it was given way to much voltage". Could you please elaborate? Knowing what happened will be a big help in fixing it.

    Cheers,
    Bruce

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    Thanks Ballen for your quick answer. Much appreciated. I hope you are right and it is a sacrificial Metal Oxide Varistor... solid German design, no doubt.

    It gave the same input and output voltage, if I remember well, about 21 volt. Now the input is about 21 volts and output 12.1 volts, as it should be? Not sure about the other outputs to the pcb. They are about 7 volts. It seems too high? I would expect 5 volts, but it could be higher because I tested without load? If you open up your displays would it be possible to also check the square connector for voltages?

    My workshop roof had water leakage. There was no direct water on it, but the humidity was quite high for a while. Not long after that it stopped responding to movements on x and y. Since that happened I fixed the roof and added a dehumidifier to keep it nice and dry.

    Could be coincidence or age?

    Anyway, vielen Dank im Voraus!

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    Hi OPW,

    Quote Originally Posted by opw View Post
    It gave the same input and output voltage, if I remember well, about 21 volt. Now the input is about 21 volts and output 12.1 volts, as it should be? Not sure about the other outputs to the pcb. They are about 7 volts. It seems too high? I would expect 5 volts, but it could be higher because I tested without load?
    That's good. I doubt that the 21V did any serious damange.

    I can have a look at the voltages inside mine, but soonest this coming weekend. Here are three questions for you meanwhile:

    As I recall, there three different unregulated DC power supplies, 5, 10 and 20 volts. Your 21 volts sounds right. So Q1 and Q2: what are the other two unregulated voltages on your unit?

    There are two different voltage regulators on the back panel. You can tell the voltage from the number, for example the MC7812 is +12V. A 7805 is +5V, a 7909 is -9V, and so on. Could you please look at the second voltage regulator, the one that you did not replace? Q3: what is its model number, and does it have the right output?

    I suggest that you unsolder and remove the MOV corpse from the board. I would also replace the blue electrolytic capacitor to the right and left and the red mylar capacitor to the left.

    Cheers,
    Bruce

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    Hey Bruce,

    The unregulated voltages I will have to get back on... But as I recall, I had only 20V and 2 x 7V. I will have to double check.

    The TO-3 regulator on the back that is not replaced (becuase I did not have it) is the MLM309K941, according to old datasheets this is a 5 Volts regulator. This output is too high I think, because when I measure directly from the regulator it showed 7 volts on both input and output. This is outside the quoted range of 4.75 - 5.25V of the datasheet. Could I replace it with a 7805?

    However, when the unit was still assembled I measured the output of the DRO to the glass scales and this was almost exactly 5V.


    Thanks for all your help!

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    Hi OPW,

    You definitely need to replace the damaged MLM309K and the fried capacitors before doing anything else. Also take the fried MOV off the board.

    What I see on the internet shows that some versions of the MLM309K can provide 1.5A with an input of up to 35 volts. In your case the input voltage is lower, so that's not a concern. The pinout is the same as a 7805, so it should be OK to substitute that, but keep in mind that most 7805 are only rated up to 1 amp. So do a good job with the heat-sink compound to get good thermal contact, and check that it does not run too warm, and I think you will be fine.

    Once you have the voltage regulator and capacitors replaced, check the unregulated supply voltages again. It would be strange to have two 7V unregulated supplies, so I suspect that one of them is meant to be a bit higher and is being pulled low by a damaged component.

    Cheers,
    Bruce

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    Thank you very much for your suggestions. I will try this and let you know what happens.

    BTW, am I right to presume that the circuit should function without the metal oxide varistor as it is connected so that it bridges from plus to minus?

    cheers,

    Oscar

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    Hi Oscar,

    Quote Originally Posted by opw View Post
    BTW, am I right to presume that the circuit should function without the metal oxide varistor as it is connected so that it bridges from plus to minus?
    Yes, that's what I am guessing. Here is the logic:

    I think the device is an MOV. If so, it will be attached between the positive and negative power supply rails, and it destroyed itself to protect the circuit downstream. So at the moment it is effectively disconnected, when you unsolder and remove it you won't change anything in the circuit from how things currently are, so won't do more damage. If this is an MOV it is only there to protect the circuit, and since you're going to be careful replacing the voltage regulator and other parts, it won't be needed for now.

    Can you verify that it's connected between the power supply plus and minus? After you've removed the device, take some close-up photos and post them here. If there are any identifying marks left, we can use those to make a positive ID.

    Cheers,
    Bruce

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    This weekend I hopefully will have the time to try some of your suggestions. The 5V regulators I ordered from Kessler Electronics, fingers crossed that they arrive on time (they usually do ship pretty fast).

    I did contact Heidenhain to see if they were willing to help with schematics or naming to component. Off course this is obsolete equipment and hobbyists like me are not their target audience. However they did make the effort of finding out what the component was and in their opinion it was a 3.3 µF 25v ceramic capacitor (with minus facing down if unit held upright). No luck on the schematics however...

    Still having some doubts though about this component though, even if directly from Heidenhain. Do ceramic caps even have polarity? The burned remains look a lot bigger then the typical 25v 3.3µF capacitor in my limited experience.
    What is your opinion on this, Bruce?


    Time for some fact-searching and measuring when I get this done, I will get back to you and post my findings.

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    Could that component have been a tantalum capacitor? They tend to disintegrate like that, not always for good reason. They are polarized, and are shaped similar to that pile of ash.

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    Hi Oscar,

    Quote Originally Posted by opw View Post
    The 5V regulators I ordered from Kessler Electronics
    I also buy stuff from them. Fast, inexpensive, and reliable.

    in their opinion it was a 3.3 µF 25v ceramic capacitor (with minus facing down if unit held upright)
    Fantastic that they gave you that info. Very useful!

    Still having some doubts though about this component though, even if directly from Heidenhain. Do ceramic caps even have polarity? The burned remains look a lot bigger then the typical 25v 3.3µF capacitor in my limited experience.
    I agree with what Kempsmith has written, this might be a tantalum capacitor. They have fallen out of popularity because they don't age gracefully and tend to self-destruct. Best thing to do is to trace the power supply circuitry. If it's a 3.3uF tantalum capacitor, it's almost certainly there for power supply decoupling. So you'll find it attached across the Vcc and ground connections of the nearest IC or ICs. If that's how it's connected, then replace it with a 3.3uF min 25volt capacitor. If you have a non-tantalum capacitor, it's probably enough to parallel it with a 100nF mylar or ceramic capacitor, to ensure that it has good high-frequency response. Alternatively replace it with a multi-layer ceramic cap, or a low ESR electrolytic in parallel with a 100nF ceramic cap.

    When I get this done, I will get back to you and post my findings.
    Very good, I look forward to hearing about it!

    Cheers,
    Bruce

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    Thanks guys for the input. Parts (regulators) have not arrived yet.

    Ballen, I guess this is the one? (Tantalelko)Just ordered it, thanks Kempsmith2



    Found this online:
    Quote:
    das Problem bereits 1982 bekannt,
    Kommt drauf an was man als Ursache annimmt.

    Die alten Tantalperlen hatten Lackumhüllung
    genau wie die Kohleschichtwiderstände
    von ehedem. Der Lack ist nicht hermetisch dicht
    wodurch langsam Feuchtigkeit eindringt was
    chemische Veränderung ( Korrosion ) begünstigt.
    Die beschleunigt sich mit erhöhter Temperatur
    ( Arrhenius ).
    I did have a humidity problem that might have accelerated the demise of the capacitor...

    Impatiently waiting for the parts...

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    Hi Oscar,

    Quote Originally Posted by opw View Post
    Ballen, I guess this is the one? (Tantalelko)
    Yup, that's right, and it's 35V rather than 25V, which should increase the reliability and decrease the chance of another melt-down. Did you trace the connections, to verify that this is connected between ground and Vcc? Are you also getting replacements for the other cooked parts?

    Looking forward to hearing if these fixes get your DRO working!

    Cheers,
    Bruce

    PS: translation of the German quote:

    The problem has been known since 1982. It's a question of what you consider responsible as the cause. The old tantalum caps had a lacquer coating, just like the metal film resistors of the day. This lacquer is not a hermetic seal so over time the humidity enters and a chemical reaction (corrosion) takes place. This is accelerated by raised temperatures.

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    Sorry, no progress. 5V regulators still not in... capacitors (ordered later) are in. Weird. German postal service screwing up? Lost in transit. Who knows...

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    Default Cutting LS803 scale

    I was thinking to open a new thread but this one was very helpful and I decide to post here my latest adventures on modifying an older Heidenhain scale, in case someone finds it usefull.
    I happen to have found an old and worn Bridgeport, usefull for nothing sort of drilling, that, however, featured a 2-axis Heidenhain DRO.
    BP has already gone, Heidenhain stayed. Then, the idea striked, that this DRO would look great on my new Chipmaster, if one circumvents the fact that the BP had a 900 mm long scale wheras the Chipmaster has only 500 mm Z travel....

    Following the advice of the very helpfull local Heidenhain seller, off I went to shorten the scale myself. If all goes well, I'll have a suitable scale for the Colch. If not, is was of no big use to me anyway.

    The idea was to mill away the aluminum profile in order to reach the glass scale, and, then, cut the scale with a diamond wheel on the Dremel. The profile should not be cut off before cutting the scale in order to provide enough support for the glass.

    Here I am milling the profile:








    Ready for the difficult part


    and success!!!




    some more milling




    So, it seems that this worked out as expected, I'll report back after testing.


    BR,
    Kyriakos

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  17. #75
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    Dear OPW,

    Quote Originally Posted by opw View Post
    Sorry, no progress.
    I had forgotten about this thread. Did you get your display working?

  18. #76
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    Hi Kyriakos,

    Nice work, as usual!

    Quote Originally Posted by dominus164 View Post
    <SNIP>
    Following the advice of the very helpful local Heidenhain seller, off I went to shorten the scale myself.
    <SNIP>
    So, it seems that this worked out as expected, I'll report back after testing.
    I have a suggestion.

    The end of the scale that you cut off probably had a reference mark (often 35mm from the end of the measuring length). You can check this by consulting the Heidenhain documentation archives for your scale, which list the number and location of reference marks.

    If you want, you can "enable" a reference mark about 35mm from your new end-of-measurement point. How do do this? Look at the scale with a jewelers loupe or magnifying glass or low power microscope. You will see "reference marks" at regular intervals. Almost all of these reference marks are "disabled" with a little spot of paint or glue. So to "enable" a reference mark near the end of your freshly-shortened scale, carefully remove the spot of paint or glue from the desired reference mark near the new end of the scale. If you fail at this, no problem, because that reference mark is not working anyway.

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  20. #77
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    Since there is so much useful info in the thread here, I'll post my problem (and solution hopefully) here:

    Subjects are the Picomax 51 Heidenhain LS 903 scales that were very dirty and needed thorough cleanup, disassembly went fine, removed the ends, carefully slid out the read head, but when I tilted the case a bit, noticed something hitting the palm of my hand wanting to escape the case, and it was the glass scale!
    So now comes the question, does anyone know if that is silicone or urethane they used to glue the glass to the anodized case? the glue is fully stuck to the glass, holds very well, the anodized surface where the scale was glued is glossy smooth, I plan to abrade that a little bit to help with the adhesion.

    What glue to use? I'll try a dab of urethane car windshield glue on the rubbery glue that is stuck to the glass to see if it would stick and hold after cure, but that glue is quite thick and will be difficult to apply evenly, so suggestions regarding adhesives are welcome

    What I would like to avoid is cyanoacrylate, had a bad experience once trying to fix a touchscreen to an lcd panel on the perimeter, there was some outgassing from the glue which condensed and stuck to the lcd/touchscreen surfaces and fogged up the screen a little near the glue points, not a big problem there, but might be fatal for these glass scales...

  21. #78
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    Quote Originally Posted by jz79 View Post
    So now comes the question, does anyone know if that is silicone or urethane they used to glue the glass to the anodized case?
    I don't know. Personally I would use silicone RTV material because it is more flexible than urethane. The thermal expansion coefficient of aluminum is about 24 x 10^-6/C and the coefficient of glass is smaller, about 5 x 10^-6. Flexibility allow for some differential thermal expansion.

    Be sure to glue it together and let it cure in a room which is 20C = 68F. Allow everything to stabilize in temperature FIRST.

    .

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    the old glue (most likely it is silicone) is still firmly stuck to the glass, my plan was to leave it there so it continues to work as the buffer for vibration and thermal expansion differences between the glass and the aluminum case

    anyway, it does seem to be some sort of a silicone, because the urethane dabs I placed on it cured and didn't adhere at all, could be easily picked off

    I ordered Dow DOWSIL 3145 link to product info pdf), from the description it looks to be the perfect stuff for this, pretty pricey, ~60eur for a small tube of 90ml, maybe some cheap sanitary silicone would also work, but I'm afraid of any unwanted additives those may contain, solvents or worse - acids etc, that might damage the deposited measuring stuff on the glass

    and as a backup I also ordered Loctite 770 primer, supposedly it lets CA glues to stick to silicone, but that will be the last resort, I'm afraid of the CA fogging up the glass scale, I could perhaps warm the glass up little bit and cool the aluminum case somewhat for any possible fumes to settle on the case, but that will be the last option

  23. #80
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    Mr. ballen
    Have read the very useful information about your Heidenhain scale adventure. An a big Thank You for sharing it.

    Of interest to me is the reference to using a IR type LED as a replacement for the scales light bulb. Wondering if you tried the so called near or the far IR type? And what were/are your thoughts on LEDs usage as a bulb replacement.

    Also would you consider sharing some more information .about the 11 micro amp to ? volt converter you made? Know that I would be interested in a schematic an how you calibrated it.. Also think other readers in the present or the future would find that information very useful.


    Thank You
    Bob


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